The erodibility of fine sediment deposits in lowland chalk streams

Abstract

Lowland chalk streams in the UK are experiencing increased deposition of fine sediment due to changes in land-use practices, channel modifications, and groundwater abstraction. The fine sediment is linked to benthic habitat degradation, the obstruction of surface-groundwater flow, and the storage of contaminants, such as nutrients and pesticides. Whilst research has been conducted on the provenance, transport, deposition, and storage of fine sediment in chalk streams, none has expressly investigated erosion. To help fill this gap in knowledge, a yearlong field survey was conducted in two reaches of the Frome-Piddle Catchment (Dorset) to quantify the erodibility of surficial fine sediment deposits. Sediment erodibility was measured in the field using a cohesive strength meter (CSM) and a shear vane. These measurements were paired with sediment cores for analysis of the physical, chemical and biological properties of the sediment. The large environmental dataset was analysed using a comprehensive suite of modern analytical techniques, including regression trees, linear regression, and mixed effects modelling. The results indicate that the erodibility of fine sediment varies significantly over time and within a stream reach due to variations in hydraulic conditions and sediment properties. Effective particle size and chlorophyll-a content were identified as the major sediment properties influencing CSM-derived erodibility, whereas root density was key for shear vane-derived strength. To date, the erosion thresholds generated by CSMs have been largely restricted to relative uses, and a calibration based on cohesive sediment is needed to permit their future incorporation into sediment transport models. This study developed an empirical calibration using laboratory experiments that estimated critical shear stress from CSM-derived erosion thresholds. By quantifying the erodibility of fine sediment deposits in chalk streams, and representing the erosion thresholds as critical shear stress, we can better gauge their local environmental impacts and help to inform models of fine sediment transport